The demand for low-sulfur heavy fuel oils is greatly increasing in recent years from the standpoint of preventing air pollution.
On the other hand, with the worldwide trend toward production of heavier crude oils, it tends to become necessary to treat heavy crude oils containing large amounts of sulfur, asphaltene, heavy metals, etc. As a result, the operation conditions of hydroconversion units at which heavy oils, e.g., atmospheric distillation residues and vacuum distillation residues, are hydrodesulfurized to low-sulfur heavy fuel oils are becoming more severe.
Moreover, since the supply of middle distillates such as kerosene and gas oil has long been insufficient for the demand therefor, the operation conditions of hydroconversion units at which heavy oils are hydrocracked are also becoming more severe.
Under such circumstances, investigations are intensively being made on the enhancement of the activity of hydroconversion catalysts and on the prolongation of the life thereof for the purpose of increasing the production of low-sulfur heavy fuel oils through the hydroconversion of heavy oils.
Also desired is the establishment of a hydrocracking technique for obtaining, with good selectivity, middle distillates of satisfactory quality from residual oils.
Hitherto, reports on processes for the hydrodesulfurization of heavy oils, e.g., atmospheric distillation residues and vacuum distillation residues, have been made by a large number of researchers.
Among these, there are a series of reports on a successive catalytic process for hydroconverting a heavy hydrocarbon oil. The processes disclosed therein employ a combination of catalysts arranged in multistage catalyst beds so as to effectively conduct the hydrodesulfurization and hydrodemetallization of a heavy hydrocarbon oil. In these processes, the heavy hydrocarbon oil is continuously hydroconverted under high-pressure conditions without partly separating the reaction products yielded in each catalyst bed.
For example, U.S. Pat. No. 4,016,067 discloses the use of a catalyst having a pore diameter of from 100 to 200 .ANG. in the first stage and a catalyst having a pore diameter of from 30 to 100 .ANG. in the second stage.
U.S. Pat. No. 4,019,976 discloses a process in which two catalysts are used for the first and second beds, respectively, with the catalyst for the second bed being made to have a higher activity in the intended reaction than the catalyst for the first bed by changing the amount of the active metal components.
U.S. Pat. No. 3,437,588 describes a process in which two or more hydroconversion catalysts having different activities are used, which catalysts are each supported on a carrier having a pore diameter of from 20 to 100 .ANG..
U.S. Pat. No. 3,977,961 describes a so-called two-stage catalyst system comprising a catalyst having a pore diameter of from 100 to 275 .vertline. which is disposed in the first stage and a catalyst having a pore diameter of from 100 to 200 .ANG. which is disposed in the second stage.
U.S. Pat. No. 3,696,027 describes a hydrodesulfurization process using classified catalysts including a catalyst having a macropore diameter (larger than 500 .ANG.).
The above prior art processes employ a down-flow type reactor packed in an upper part thereof with a catalyst having a high large-pore content and in a lower part thereof with a catalyst having a low large-pore content.
U.S. Pat. No. 3,535,225 discloses a two-stage hydrocracking process in which either or both of an alumina catalyst and a silica-alumina catalyst are used in the first stage and a zeolite catalyst is used in the second stage.
U.S. Pat. No. 3,385,781 discloses a two-stage hydrocracking process in which a zeolite having a large pore diameter is used in the first stage and a zeolite having a small pore diameter is used in the second stage.
U.S. Pat. Nos. 3,730,879, 3,766,058, and 4,048,060 describe a two-stage catalyst system in which the catalyst used in the second stage has a larger pore diameter than the catalyst used in the first stage.
In processes for the hydroconversion of heavy oils using conventional multi-stage catalyst combinations, the catalysts to be used in combination are classified according to only one factor selected from physical properties such as pore size distribution, average pore diameter, the contents of active metals, the kind of the zeolite contained, etc., as in the prior art processes described above.
However, such catalyst systems have some problems concerning the deactivation stability of the catalysts used in combination. It is therefore difficult to consistently maintain a high charge rate and a high degree of hydrocracking.
For example, if a catalyst combination is improperly designed with respect to a physical property such as pore volume, pore size distribution, or pore diameter, the catalyst system has poor deactivation stability even though the initial desulfurization activity thereof is high, resulting in a gradual decrease in the degree of hydrocracking during the latter half of operation.
In the case of a silica-alumina or zeolite catalyst, a high degree of hydrocracking is obtained initially, but this performance cannot be maintained over a relatively long time because such catalysts have poor deactivation stability.